Catch devices are already known that comprise a main casing receiving a hook that is hinged about a first axis, and a locking lever that is hinged about a second axis parallel to the first axis, one branch of the lever carrying a thrust wheel that co-operates with a camming surface of the hook, while its other branch is connected via a linkage to the outlet shaft of a driving motor and gear box assembly.
However, catch devices having the above-specified structure are used in emergency only, i.e. they are designed to be operated only in the event of a hydraulic breakdown in the normal unlocking system.
In the search for centralized power generation that is electrical rather than hydraulic, it seems advantageous to use a device of the above-specified type as the normal unlocking system.
However, the person skilled in the art tends to reject such an application because of the risks involved. There is no way of avoiding all risk of mechanical failure in the gear box (tooth breakage, sets of teeth and/or bearings seizing), and the consequences of such risks are extremely severe because if a hinged hook jams, it prevents the corresponding landing gear being lowered.
Although the probability of such a fault is low, the severity of its consequences leads the person skilled in the art to consider emergency mechanisms capable of operating in the event of a breakdown that may be local or general. It should be recalled that the objective in aviation is to limit the breakdown rate to a value of about 10.sup.-9 per hour of operation.
The person skilled in the art could consider fitting the gear box with a differential system, or could consider using two separate gear boxes, however neither of those solutions appears to be entirely satisfactory from the objective point of view, and they also have undesirable effects on mass and bulk.